fallocate mode flag for "unshare blocks"?

[Christoph Hellwig]

On Wed, Mar 30, 2016 at 05:32:42PM -0700, Liu Bo wrote:
> Well, btrfs fallocate doesn't allocate space if it's a shared one
> because it thinks the space is already allocated.  So a later overwrite
> over this shared extent may hit enospc errors.

And this makes it an incorrect implementation of posix_fallocate,
which glibcs implements using fallocate if available.